Ferrofluid-type seal apparatuses for use in sealing rotary shafts, with single- and multiple-stage, ferrofluid-liquid, O-ring seals about the shaft, are well known (see, for example, U.S. Pat. No. 3,620,584 which describes a multiple-stage, ferrofluid, rotary-shaft seal).
Single- and multiple-stage ferrofluid seals have been used as exclusion seals, to protect one environment on one side of the shaft from contaminants in an environment on the other side of the shaft. Ferrofluid-type exclusion seals are useful particularly with computer-disc-drive spindles, to prevent contaminants in an environment from reaching a memory-disc area.
One standard ferrofluid exclusion seal presently employed in the computer field comprises an annular, ring-like, permanent magnet adapted to surround the spindle shaft and sandwiched between two, identical, pole-piece elements which are placed at the outer diameter into a contacting, magnetic-flux relationship with the one and the other polar ends of the permanent magnet. The inner diameter of the pole-piece elements extends into a close, noncontacting relationship with the surface of the shaft or spindle, to form a small gap for example, 2 to 10 mils, between the inner diameter of the pole piece elements and the shaft surface. A ferrofluid is disposed and magnetically retained in the gaps on the insertion of the magnetically permeable shaft or spindle, to form one or more liquid O-ring stages, which serve to form a ferrofluid exclusion seal about the shaft.
A wide variety of magnetic materials may be used to provide the permanent magnet, but usually the material is a sintered or bonded ceramic material having a longitudinal thickness of about 80 to 150 mils. The pole-piece elements are composed of magnetically permeable material, such as magnetic stainless steel (for example, 400 series), and range in thickness from about 25 to 80 mils. The standard exclusion seal, depending on customer requirements, is provided as described or placed in a nonmagnetic housing, such as of aluminum or stainless steel (for example, 300 series), such as by bonding- or staking-assembly techniques.
The exclusion seal is formed by placing a precise, optimum amount of a ferrofluid in the annular gap regions between the inner diameter of the pole pieces and the spindle shaft. Typically, the ferrofluid comprises a low-vapor-pressure carrier liquid, such as a fluorocarbon, a polyphenylether, a hydrocarbon, a diester liquid and similar low-vapor-pressure liquids, to provide for a very low mass loss of the ferrofluid forming the O-ring seal, thereby providing an exclusion seal of long operating life. For example, the standard ferrofluid exclusion seal is expected generally to last for several years under moderate temperature conditions and with the currently used computer-disc-drive-spindle speeds of 3600 rpm and with shaft diameters up to about 1.8 inches. The ferrofluid used may vary in viscosity, and the saturation magnetization, which usually ranges from 20 to 500 cps, and 100 to 400 gauss respectively.
It is desirable to extend the useful operating life of ferrofluid exclusion seals, particularly under higher ambient-temperature conditions; for example, greater than 50.degree. C., at spindle speeds that exceed 3600 rpm, and for larger shaft diameters, or a combination of these conditions.